Chiral binuclear metal complexes for stereoselective hydrolysis of saccharides and glycosides

ABSTRACT

Disclosed herein is a class of chiral binuclear metal complexes for stereoselective hydrolysis of saccharides and glycosides, and more particular chiral binuclear transition metal complex catalysts that discriminate epimeric glycosides and α- and β-glycosidic bonds of saccharides in aqueous solutions at near physiological pHs. The chiral binuclear metal complexes include a Schiff-base-type ligand derived from a chiral diamino building block, and a binuclear transition metal core, each which can be varied for selectivity. The metal core is a Lewis-acidic metal ion, such as copper, zinc, lanthanum, iron and nickel. The Schiff-base may be a reduced or non-reduced Schiff-base derived from aliphatic linear, aliphatic cyclic diamino alcohols or aromatic aldehydes. The ligand can be a penta- or heptadentate ligand derived from pyridinecarbaldehydes, benzaldehydes, linear or cyclic diamines or diamino alcohols.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a divisional application ofU.S. patent application Ser. No. 15/451,478 filed Mar. 7, 2017, whichclaims priority to and the benefit of U.S. Provisional PatentApplication Ser. No. 62/312,873, each of which is incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No.CHE-1305543, awarded by the National Science Foundation. The U.S.government has certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates generally to chiral binuclear transition metalcomplexes for stereoselective hydrolysis of saccharides and glycosides,and more particular to chiral binuclear Lewis acid transition metalcatalysts that discriminate epimeric glycosides and α- and β-glycosidicbonds of saccharides in aqueous solutions at near physiological pHs.

2. Description of the Related Art

Chiral discrimination is displayed by many enzyme classes, such asglycosidases, lipases, and esterases during bond formation and cleavage.While much progress has been made in the synthesis and evaluation ofselective catalysts mimicking such features, most synthetic entities donot show sufficient diastereoselective or even chiral discriminationability in aqueous solution. Man-made stereoselective catalysts for thediscrimination of epimeric glycosides in aqueous solution or theselective hydrolysis of α- and β-glycosidic bonds are not previouslyknown.

A symmetric binuclear copper(II) complex,N,N′-{1,3-bis[(pyridine-2-ylmethyl) amino]propan-2-al}ato dicopper(II)(μ-acetato) diperchlorate, was previously observed to interactdifferently with glycopyranosides upon binding in alkaline solutionresulting in a 30-fold stronger binding to mannose over glucose. Similarobservations were subsequently made using related metal complexesconfirming the results of saccharide-metal complex binding in alkalineaqueous solution.

It is therefore desirable to provide chiral binuclear metal complexesfor selective glycoside hydrolysis.

It is further desirable to provide chiral binuclear transition metalcomplexes for stereoselective hydrolysis of saccharides.

It is yet further desirable to provide asymmetrical binuclear Lewis acidtransition metal complexes that discriminate epimeric saccharides duringhydrolysis enabling chemoselective catalysis.

It is still yet further desirable to provide chiral binuclear Lewis acidtransition metal complex catalysts that discriminate epimeric glycosidesand aryl α- and βglycosidic bonds of saccharides in aqueous solutions atnear physiological pHs

It is still yet further desirable to provide chiral binuclear transitionmetal complex catalysts that at near neutral pH discriminate epimericglycosides (e.g., α-mannoside, α-galactoside and α-glucoside).

It is still yet further desirable to provide chiral binuclear transitionmetal complex catalysts that at near neutral pH discriminatedisaccharides with different glycosidic bonds (e.g., maltose andcellobiose).

It is still yet further desirable to provide chiral binuclear transitionmetal complex catalysts that at near neutral pH discriminate epimericdisaccharides with identical glycosidic bonds (e.g., lactose andcellobiose).

It is still yet further desirable to provide chiral binuclear metalcomplexes that promote the development of catalysts to replace orcomplement slow-activating natural glycosidases.

It is still yet further desirable to provide chiral binuclear metalcomplexes that are advantageously easy to prepare including in largescale synthesis, easy to store at ambient temperatures, have unlimitedshelf-life without a decrease in activity, superior stability againstpH, temperature, solvents and/or aging, and insensitivity to poisoningby transition metal ion traces.

It is still yet further desirable to provide chiral binuclear Lewis acidtransition metal complex catalysts to hydrolyze and/or digestglucosides, disaccharides and oligosaccharides selectively fromindustrial waste in food industry and/or from biomass.

Other advantages and features will be apparent from the followingdescription, and from the claims.

SUMMARY OF THE INVENTION

In general, in a first aspect, the invention relates to a chiralbinuclear transition metal complex for stereoselective hydrolysis ofsaccharides and glycosides. The chiral binuclear transition metalcomplex includes a Schiff-base ligand derived from a chiral diaminobuilding block; and a binuclear transition metal core. The chiraldiamino building block may be varied by changing the distance of themetals of the binuclear core in order to tune the complexes selectivity.The Schiff-base may be a reduced or non-reduced Schiff-base derived fromaliphatic linear, aliphatic cyclic diamino alcohols or aromaticaldehydes. The complex is formulated to discriminate epimeric glycosidesand α- and β-glycosidic bonds of saccharides in aqueous solutions atnear physiological pHs. The metal core is a Lewis-acidic metal ion, suchas copper, zinc, lanthanum, iron and nickel. The ligand can be a penta-or heptadentate ligand derived from pyridinecarbaldehydes,benzaldehydes, and linear or cyclic diamines or diamino alcohols, suchas:

In general, in a second aspect, the invention relates to a chiralbinuclear transition metal (II) complex having the formula [M₂L_(-H)]³⁺,wherein L=S- or R-N,N′-1,3-bis[(pyridine-2-ylmethyl)amino]propan-2-al,wherein M is a Lewis-acidic metal ion selected from the group consistingof copper, zinc, lanthanum, iron and nickel. In particular, the complexmay have the formula of C₁₉H₂₈Cl₂Cu₂N₄O₁₂, namely 2S,N,N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]ato dicopper(II)(μ-acetato) diperchlorate, 2R,N,N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]ato dicopper(II)(μ-acetato) diperchlorate or a combination thereof. Moreover, thecomplex can be:

In general, in a third aspect, the invention relates to a method ofhydrolysis of saccharides and glycosides. The method includesdiscriminating epimeric glycosides and α- and β-glycosidic bonds ofsaccharides in aqueous solutions at near physiological pHs using achiral binuclear transition metal (II) complex. The S-2 enantiomerdiscriminates α-glycosidic bonds of the saccharides, and the R-2enantiomer discriminates β-glycosidic bonds of the saccharides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A graphically illustrates the catalytic proficiency of binuclearcomplexes 1, S-1 and R-1 in aqueous solution at pH 10.5 during thehydrolysis of 4-nitrophenylglycosides, and in particular graphicallyillustrates catalysts promoting the hydrolysis of glycosides as 8a-f;

FIG. 1B graphically illustrates the catalytic proficiency of binuclearcomplexes 1, S-1 and R-1 in aqueous solution at pH 10.5 during thehydrolysis of 4-nitrophenylglycosides, and in particular graphicallyillustrates glycosides 8a-f hydrolyzed by catalysts 1, S-1 and R-1;

FIG. 2 graphically illustrates the product formation over time, catalystand substrate concentration for the hydrolysis of 9a, 9c, and 9e by S-1in 50 mM HEPES buffer at 7.50±0.05 and 30.0±0.1° C.;

FIG. 3 graphically illustrates the proficiency of S-1 for the catalytichydrolysis of α-glycosides 9a, 9c, and 9e in 50 mM HEPES buffer at pH7.50±0.05 and 30.0±0.1 ° C.;

FIG. 4 graphically illustrates product formation during the hydrolysisof 9e and 9f by S-1 in 50 mM HEPES buffer at pH 7.50±0.05 and 30.0±0.1°C.;

FIG. 5A illustrates various disaccharide structures in accordance withan illustrative embodiment of the invention disclosed herein;

FIG. 5B graphically illustrates catalyzed disaccharide hydrolysis inaccordance with an illustrative embodiment of the invention disclosedherein; and

FIG. 6 graphically illustrates catalyzed hydrolysis of2-chloro-4-nitrophenyl-α-D-mannopyranoside at pH 7.5 in 50 mM HEPESbuffer; Cu₂bpdpo (red) (k_(cat)=8.1±0.9 10⁻⁴ min⁻¹; K_(M)=12.0±2.3 mM);Zn₂tpdpo (blue) (k_(cat)=6.4±0.6×10⁻⁴min⁻¹; K_(M)=11.0±1.6 mM;k_(non)=4.4×10⁻⁷min⁻¹N⁻¹).

DETAILED DESCRIPTION OF THE INVENTION

The compounds and methods discussed herein are merely illustrative ofspecific manners in which to make and use this invention and are not tobe interpreted as limiting in scope.

While the compounds and methods have been described with a certaindegree of particularity, it is to be noted that many variations andmodifications may be made in the details of the sequence, synthesis,components, concentrations and the arrangement of the processes andcompositions without departing from the scope of this disclosure. It isunderstood that the compounds and methods are not limited to theembodiments set forth herein for purposes of exemplification.

The chiral binuclear metal complexes disclosed herein include aSchiff-base ligand derived from a chiral diamino building block, and abinuclear transition metal core, each which can all be varied. Thechiral binuclear metal complexes may have formulasN,N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]ato dicopper(II)(μ-acetato) diperchlorate, (racemic complex 2 below), 2S,N,N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]ato dicopper(II)(μ-acetato) diperchlorate, (“S-Cu₂bpdbo” or chiral complex S-2 below)and 2R, N,N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]atodicopper(II) (μ-acetato) diperchlorate, (“R-Cu₂bpdbo” or chiral complexR-2 below), as shown by the following structural formulas as:

The chiral binuclear metal complexes can be synthesized from variousLewis acidic metals ions, such as Zn²⁺, La³⁺, Fe³⁺, or Ni²⁺, such as:

In addition, the chiral diamino building block can be varied by changingthe distance of the metals of the binuclear core in order to tune thecomplexes selectivity. For example, the Schiff-base may be reduced ornon-reduced and be derived from aliphatic linear, aliphatic cyclicdiamino alcohols or aromatic aldehydes. The ligands can be penta- andheptadentates derived from pyridinecarbaldehydes, benzaldehydes, andlinear or cyclic diamines and diamino alcohols, such as:

EXAMPLES

The chiral binuclear metal complexes disclosed herein are furtherillustrated by the following examples, which are provided for thepurpose of demonstration rather than limitation. Although copper andzinc were used in the following examples due to their relatively lowcost and high functionality, a number of other Lewis-acidic metals canbe used.

As demonstrated by the following examples, chiral binuclear copper(II)and zinc (II) complexes S-1 and R-1 were synthesized and fullycharacterized including analysis by X-ray diffraction to confirm theirstereochemistry. Subsequent evaluations of the complexes as catalystsfor the cleavage of glycosidic bonds in aqueous alkaline solution showedmoderate proficiency during the hydrolysis of 4-nitrophenyl glycosidesand small discrimination ability among the selected epimeric substrates.

However, at near physiological pH, chiral complex S-1 shows distinctdiscrimination of epimeric aryl α-glycopyranosides. Discrimination of α-and β-glycosidic bonds in manno- and galactopyranosides by the samecomplex is apparent in their reaction rates, but masked in the catalyticproficiency by the different rates of the uncatalyzed reaction. Bycontrast, a 28-fold faster hydrolysis of aryl β- over α-glucopyranosideis noted translating into 3-fold higher proficiency of chiral complexS-1 for the hydrolysis of β-glucopyranoside, while the uncatalyzedreactions are of the same order of magnitude for both substrates. Thediscrimination is not related to the chirality of the complexes, butrather due to the configuration of the glycosides promoting cis- ortrans-configured diol binding sites for catalyst coordination.Mechanistic studies reveal deprotonation of the hydroxyl group at C-2 aspre-requisite for catalysis.

An initial catalyst evaluation toward the hydrolysis of representativedisaccharides revealed a preference of chiral complex S-1 for thecleavage of α-glycosidic bonds, and of chiral complex R-1 for thehydrolysis of β-glycosidic bonds. The chiral complexes S-1 and R-1 arecatalysts able to discriminate epimeric and anomeric model glycosidespromoting a stereoselective hydrolysis of glycosidic bonds insaccharides in aqueous solution at near neutral or physiological pH. Thechiral catalysts S-1 and R-1 catalysts may be further developed for usewith biomass transformation into valuable chemical synthons and fuels,applications in pharmaceutical industry, and/or the development offunctional enzyme mimics.

Example 1 Synthesis of Chiral Binuclear Copper(II) Complexes

Symmetric complex N,N′-{1,3-bis[(pyridin-2-ylmethyl)amino]butan-2-al}atodicopper(II) (μ-acetato) diperchlorate, (“Cu₂bpdpo”) (symmetric complex1 above) was derived from a reduced Schiff-base, free amino ligand,namely N,N′-1,3-bis[(pyridine-2-ylmethyl)amino]propan-2-al (“bpdpo”)that was obtained from 1,3-diaminopropanol and pyridinecarbaldehyde.

Similarly, chiral complexes S-Cu₂bpdbo S-1 and R-Cu₂bpdbo R-1 wereprepared according to Scheme 1 below by using enantiopure S- and R-malicacid 2 as inexpensive starting material for the synthesis of chiral1,4-diaminobutanols 3. In short, the chiral S- and R-malic acid 4 wereconverted into methyl malates 5 using methanol in the presence of acetylbromide. Treatment of esters 5 with excess ammonia in methanol yieldedmalamides 6. The hydrochlorides of S-3 and R-3 were obtained afterreduction of malamides 6 with borane in THF and treatment of thereaction products with hydrogen chloride in absolute ethanol.Condensation of the free diaminoalcohols with pyridinecarbaldehydeafforded the chiral ligands S-bpdbo (5-7), and R-bpdbo (R-7),respectively, after reduction of the initially formed Schiff bases withsodium borohydride in methanol. Enantiopure binuclear copper(II)complexes S-1 and R-1 were prepared from the pentadentate ligands 7 andcopper(II) acetate in methanol.

Acetyl bromide (2.2 mL, 0.031 mol) was added dropwise to 20 mL coldmethanol. The solution was stirred for 30 minutes in ice. Then R-malicacid R-3, (12.5 g, 93.28 mmol) was added. The acid dissolved in about 5min, and the resulting solution was stirred at ambient temperature.After 18 h, sodium bicarbonate (4.00 g, 0.048 mol) was added to the paleyellow solution. After 15 min of stirring, the mixture was filtered, thefiltrate collected, and all volatile compounds evaporated in vacuum,yielding a sticky, oily raw material containing a white precipitate. Thedesired ester was distilled from this mixture in vacuum yielding 8.88 gof (R)-methyl malate (R-5) (54.77 mmol, 59%) of a colorless liquid.

The foregoing process was replicated using S-malic acid (S-3) in orderto produce (S)-methyl malate (S-5).

(S)-methyl malate S-5 (6.20 g, 38.27 mmol) was then dissolved in 45 mLof 7 N ammonia in methanol under inert atmosphere and stirred at ambienttemperature. After 24 h, the formed precipitate was separated byfiltration and washed with methanol. The raw material was recrystallizedfrom methanol and dried in vacuum yielding 2.20 g of (S)-malamide (S-6)(16.54 mmol, 43%) as a colorless solid.

(R)-malamide (R-6) was prepared from 6.84 g (42.22 mmol) of R-methylmalate R-5, as described for S-malamide from S-5, yielding 2.68 g (20.15mmol, 48%) of R-6 as a colorless solid.

Under an inert atmosphere, 100 mL of 1 M borane in THF were added toice-cooled S-6 (1.50 g, 11.28 mmol), and the resulting solution washeated to 77° C. After 11 h, the solution was cooled in an ice bath, and40 mL of methanol were added in small portions to control the gasdevelopment. The solution was then heated to reflux for 1 h. Aftercooling, all volatile components were removed by rotary evaporationleaving the crude material as yellowish oil. The oil was dried in vacuumyielding a gummy-like off-white solid that was triturated with 200 mLwater-free ethanol and filtered. The filtrate was subjected to gaseousHCl in the cold yielding a precipitate. The precipitate was isolated,washed once with 2 mL ice-cold ethanol and dried in vacuum over drieriteyielding 0.575 g of (S)-1,4-diamino-2-butanol hydrochloride (S-4).

(R)-1,4-diamino-2-butanol hydrochloride (R-3) was obtained as colorlesssolid in 74% yield (5.96 g, 33.67 mmol) from 6.0 g (44.8 mmol)(R)-malamide R-6 as described above for the synthesis of S-3 from S-6.

Sodium hydroxide (1.53 g, 38.25 mmol) was added to a solution of S-4(1.50 g, 8.475 mmol) in 80 mL methanol at ambient temperature. Theinitially turbid solution became clear and after 5 min turbid again.After 5 h, 2.27 g (21.19 mmol) of distilled 2-pyridinecarbaldehyde wereadded. After additional 22 h, the solution was diluted with 80 mLmethanol prior to the addition of 3.04 g (0.080 mol) of sodiumborohydride. After further 48 h, all volatile material was removed invacuum to yield a residue that was taken up in 50 mL chloroform and 15mL ice water. The organic layer was separated and extracted two timeswith 15 mL of ice water each. The combined organic layer was dried oversodium sulfate, filtered and concentrated to dryness yielding 2S, N,N′-bis(2-pyridylmethyl)-1,4-diaminobutan-2-ol, (S-bpdbo) (S-7) as ayellowish oil (1.42 g, 4.958 mmol, 59%). Typically, the S-7 ligandobtained by this procedure was diluted in an appropriate amount ofethanol to yield a 1 M stock solution, which was then used withoutfurther purification or characterization for the synthesis of copper(II)complexes. To obtain analytical data, 1.0 g (3.491 mmol) of the raw S-7material were dissolved in dichloromethane and purified by columnchromatography over silica gel (dichloromethane/methanol, 20/1-1/1, v/v)yielding 0.52 g (1.815 mmol, 52%) of S-7 as a pale yellowish oil.

2R, N, N′-bis(2-pyridylmethyl)-1,4-diaminobutan-2-ol, (R-bpdbo)(R-7) wasprepared from R-6 using the same procedure as described for thesynthesis of S-7 from S-6 above yielding 2.42 g (8.450, quantitative) ofR-7 as raw material. Purification of the raw material by columnchromatography over silica gel (dichloromethane/methanol, 20/1-1/1, v/v)yielded 2.01 g (7.0189 mmol, 83%) of R-7 as a pale yellowish oil.

Copper(II) acetate monohydrate (2.00 g, 10.00 mmol) were dissolved in 20mL water and 400 mL methanol at ambient temperature. To thegreenish-blue solution, 4.5 mL of the 1 M stock solution of S-7 inethanol was added followed by a solution of 4.00 g (32.68 mmol) ofsodium perchlorate in 10 mL water and 40 mL ethanol. The resulting darkblue solution was stirred for 12 h, filtered and concentrated below 50°C. to about 40 mL. Upon standing at ambient temperature, a precipitateformed that was isolated by filtration and dried at ambient temperaturein air yielding 2.85 g (4.155 mmol) of a blue raw material. The rawmaterial was recrystallized from aqueous methanol yielding 2.19 g (3.193mmol, 32%) of 2S, N,N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]ato dicopper(II)(μ-acetato) diperchlorate, ((S)-Cu₂bpdbo)(S-1) as a blue solid.

2R, N, N′-[1,4-bis[(pyridin-2-ylmethyl)amino]butan-2-ol]ato dicopper(II)(μ-acetato) diperchlorate, (R-Cu₂bpdbo)(R-1), was prepared from 4.5 mLof a 1 M stock solution of R-77 as described for the preparation ofS-Cu₂bpdbo from S-7 yielding 1.70 g (2.478 mmol, 25%) of R-1 as a bluesolid.

Crystals of chiral complexes S-2 and R-2 were mounted on MiTeGencryoloops in random orientations. Preliminary examination and datacollection were performed using a Bruker X8 Kappa Apex II Charge CoupledDevice (CCD) Detector system single crystal X-Ray diffractometerequipped with an Oxford Cryostream LT device. All data were collectedusing graphite monochromated Mo Kα radiation (λ=0.71073 Å) from a finefocus sealed tube X-Ray source. Preliminary unit cell constants weredetermined with a set of 36 narrow frame scans. Typical data setsconsist of combinations of ω and ϕ scan frames with a scan width of 0.5°and counting time of 15 seconds/frame at a crystal to detector distanceof 4.0 cm. The collected frames were integrated using an orientationmatrix determined from the narrow frame scans. Apex II and SAINTsoftware packages were used for data collection and dataintegration.[50] Analysis of the integrated data did not show any decay.Final cell constants were determined by global refinement of reflectionsharvested from the complete data set. Collected data were corrected forsystematic errors using SADABS based on the Laue symmetry usingequivalent reflections. The disorder was modeled with partial occupancyatoms and geometrical restraints for both structures.

Structure solution and refinement were carried out using theSHELXTL-PLUS software package. The structures were solved by directmethods and refined successfully in the space group P2₁2₁2₁. Full matrixleast-squares refinements were carried out by minimizing Σw(F₀ ²-F_(c)²)². The non-hydrogen atoms were refined anisotropically to convergence.All hydrogen atoms were treated using appropriate riding models.

X-ray data for chiral complex S-2 (C₁₉H₂₈Cl₂Cu₂N₄O₁₂), blue needles(0.567×0.149×0.102 mm3, V 2640.4(3) Å3), were collected at 100 K. Thecrystals are orthorhombic, space group P2₁2₁2₁ with a=7.2680(5) Å,b=14.5809(10) Å and c=24.9155(17) Å, and Z=4. The θ-range for datacollection was 1.618 to 30.743°. The number of reflections collected was61147, with 8157 unique reflections (R_(int)=0.0402). Refinement byfull-matrix least-squares on F², 392 parameters, gave final R indices(I>2σ_(I)) R₁=0.0290, weighted R₂=0.0706; R indices on all data wereR₁=0.0335 and weighted R₂=0.0721. The absolute structure parameter x was−0.020(3); CCDC 1431768.

X-ray data for chiral complex R-2 (C₁₉H₂₈Cl₂Cu₂N₄O₁₂), blue needles(0.516×0.104×0.066 mm3, V 2650.10(13) Å3), were collected at 100 K. Thecrystals are orthorhombic, space group P2₁2₁2₁ with a=7.2854(2) Å,b=14.5841(4) Å and c=24.9419(7) Å, and Z=4. The θ-range for datacollection was 1.633 to 34.970°. The number of reflections collected was88769, with 11457 unique reflections (R_(int)=0.0662). Refinement byfull-matrix least-squares on F², 385 parameters, gave final R indices(I>2σ_(I)) R₁=0.0398, weighted R₂=0.0832; R indices on all data wereR₁=0.0570 and weighted R₂=0.0891. The absolute structure parameter x was−0.021(5); CCDC 1431767.

Example 2 Characterization of Chiral Binuclear Copper(II) Complexes inSolution

To evaluate the chiral binuclear metal complexes for their ability todiscriminate glycosidic bonds, two pH values for kinetic evaluationswere selected reflecting different complex compositions. The compositionof chiral complexes S-1 and R-1 in aqueous solution is identical to thespeciation of racemic complex 1 and deduced from data previouslydetermined using spectrophotometric titration methods. At pH 10.5, thepredominant species of chiral complexes S-1 and R-1 is still a[Cu₂L_(-H)]³⁺ species (57.5%), while only one other additional[Cu₂L_(-H) (OH)]²⁺ species (42.5%) is formed (L=S- and R-bpdbo,respectively) (Scheme 2 below, X=H₂O). By contrast, symmetric complex 1forms under these conditions [Cu₂L_(-H) (OH)₂]⁺ as the main species(88.5%) in equilibrium with a minor [Cu₂L_(-H)(OH)]²⁺ species (11.5%).Mononuclear complexes formed from remaining ligand or free metal ionscan be neglected for catalysis under the applied conditions or weredemonstrated to be inactive. At pH 7.5, chiral complexes S-1 and R-1exist predominantly as a binuclear [Cu₂L_(-H)]³⁺ species (94%, L=S- andR-bpdbo, respectively), while symmetric complex 1 exists as[Cu₂L_(-H)(OH)]²⁺ species (98.6%, L=bpdpo) (Scheme 3, X=H₂O).

To allow a comparison between all complexes and species derivedtherefrom during the hydrolysis of glycosidic bonds, the catalystamounts used for the determination of kinetic parameters are correctedto reflect the different amounts of the respective catalytically activespecies.

Example 3 Differentiation of 4-nitrophenyl Glycosides During CatalyticHydrolysis in Alkaline Solution

Based on previously established assays using UV/Vis spectroscopy toevaluate the catalytic activity of symmetric complex 1 and racemiccomplex 2 during glycoside hydrolysis, chiral catalysts S-1 and R-1 wereextended and the substrate scope by employing six commercially available4-nitrophenylglycosides, and transferred the previous assay from 1 mLstandard cuvettes into 96-well plate format. The adjusted procedureallows considerably faster catalyst screening using smaller compoundamounts and volumes, and thereby circumvents previously observedlimitations caused by low substrate solubility.

Along these lines, the catalytic hydrolysis of4-nitrophenyl-α-D-mannopyranoside (8a),4-nitrophenyl-β-D-mannopyranoside (8b),4-nitrophenyl-α-D-galactopyranoside (8c),4-nitrophenyl-β-D-galactopyranoside (8d),4-nitrophenyl-α-D-glucopyranoside (8e), and4-nitrophenyl-β-D-glucopyranoside (8f), whose structural formulas areshown below, was studied by UV/Vis spectroscopy recording the productformation at 405 nm over time pursuant to Scheme 4.

Catalytic hydrolysis of 8e is illustrated in Scheme 4 below asrepresentative example for the hydrolysis of substrates 8a-f

The adjusted assay has a total volume of 200 μL and lowered the catalystconcentration from 0.1 mM to 0.03 mM, while the substrates were usedbetween 6-10 mM. The substrate hydrolysis depends linearly on thecatalyst concentration under these conditions. The measured absorbancewas converted into concentration using the apparent extinctioncoefficient Capp, corrected for the catalyst concentration, its relativespeciation amount, and the uncatalyzed reaction, and then plotted versusthe substrate concentration. By applying a non-linear fit to theresulting hyperbolic data, the catalytic rate constant k_(cat) [min⁻¹]and the substrate affinity K_(M) [mM] were determined utilizing theMichaelis-Menten model (Table 1).

TABLE 1 Kinetic parameters for the hydrolysis of 4-nitrophenylglycosides8a-f at pH 10.5 and 30° C. k_(cat) × 10⁻³ K_(M) k_(cat)/K_(M) × 10⁻³k_(cat)/k_(non) × 10⁻³ k_(cat)/(K_(M) × Entry S cat [min⁻¹] [mM] [min⁻¹M⁻¹] [M] k_(non)) 1 8a 1 3.09 107.5 28.7 20.7 193,000 2 S-1 6.32 107.758.7 42.4 394,000 3 R-1 4.42 57.7 76.6 29.7 514,000 4 8b 1 0.30 21.014.3 1.5 74,000 5 S-1 0.76 26.6 28.6 3.9 147,000 6 R-1 0.64 23.9 26.73.3 138,000 7 8c 1 0.65 70.5 9.2 18.2 258,000 8 S-1 1.07 42.1 25.4 29.9710,000 9 R-1 1.15 18.7 61.7 32.1 1,720,000 10 8d 1 0.12 12.1 9.9 0.974,000 11 S-1 0.28 10.2 27.5 2.1 206,000 12 R-1 0.26 16.8 15.5 1.9116,000 13 8e 1 0.27 7.5 35.8 2.5 335,000 14 S-1 0.83 9.1 91.5 7.8857,000 15 R-1 0.81 6.3 129.0 7.6 1,210,000 16 8f 1 0.15 53.2 2.8 0.313,000 17 S-1 0.81 72.2 11.2 1.4 51,000 18 R-1 1.17 42.8 27.3 20.0124,000 where k_(non), 8a = 1.5 × 10⁻⁷ [min⁻¹ M⁻¹], k_(non), 8b = 1.9 ×10⁻⁷ [min⁻¹ M⁻¹], k_(non), 8c = 0.4 × 10⁻⁷ [min⁻¹ M⁻¹] , k_(non), 8d =1.3 × 10⁻⁷ [min⁻¹ M⁻¹]; k_(non), 8e = 1.1 × 10⁻⁷ [min⁻¹ M⁻¹], k_(non),8f = 2.2 × 10⁻⁷ [min⁻¹ M⁻¹]

The uncatalyzed reactions (k_(non)) of all substrates remain in the sameorder of magnitude as previously determined (k_(non)=0.4-2.2×10⁻⁷ min⁻¹M⁻¹). For comparison of different substrates, only the proficiency(k_(cat)/(K_(M)×k_(non))) of the catalysts is discussed to account forthe different strengths of glycosidic bonds and the resulting differenthydrolysis rates in absence and presence of catalysts.

The catalytic proficiency of the symmetric complex 1 under the employedconditions is very modest and shows only limited differentiation ofsubstrates with α-glycosidic bond (8a, 8c and 8e) from substrates withβ-glycosidic bond (8b, 8d and 8f) that are typically hydrolyzed evenless efficiently, if at all (FIG. 1). The substrates are notsignificantly discriminated by symmetric complex 1 for their epimericsugar moiety.

By contrast, both chiral complexes S-1 and R-1 are more proficient forcleaving the glycosides 8a-f than symmetric complex 1 (FIG. 1A). Inaddition, chiral complexes S-1 and R-1 show higher proficiency thansymmetric complex 1 to hydrolyze α-over β-glycosidic bonds. Chiralcomplex R-1 hydrolyzes 8c with a 6.7-fold, and 8e with a 3.6-fold higherproficiency than symmetric complex 1, and shows a 1.3-fold, and2.4-fold, respectively, higher proficiency for the hydrolysis of thesame substrates than chiral complex S-1. While chiral complex R-1 showsthe overall highest proficiency for hydrolyzing4-nitrophenyl-α-D-galactopyranoside (8c)(k_(cat)/K_(M)×k_(non)=1,720,000), the same complex shows an almost10-fold higher proficiency for hydrolyzing the β-glucosidic bond in 8f(R-1: k_(cat)/K_(M)×k_(non)=124,000) than symmetric complex 1k_(cat)/K_(M)×k_(non)=12,800).

While the results encourage further investigation of binuclear Cu(II)complexes S-1 and R-1 for their ability to discriminate glycosidic bondsin natural systems, including disaccharides, initial attempts to usethese substrates under the above described assay conditions were futileand led to catalyst destruction. Visibly to the naked eye, theoriginally blue solutions will turn green and then orange within 1-2 hindicating the formation of Cu(I) oxide without evidence for asignificant hydrolysis of the disaccharide at 30 or 40° C. Decreasingthe pH of the solution to pH 7 or 8 was found to increase catalyststability, but resulted in catalyst inactivation below 40° C. (seebelow). Selective oxidation of the primary hydroxyl group at C-6 inmethyl glycosides was previously observed for symmetric complex 1 afteractivation with TEMPO in alkaline solution.

Following the hydrolysis of glycosides and natural saccharides at nearphysiological pH appears more relevant for the synthesis of functionalenzyme models and modelling of enzyme activity. Unfortunately, thecommercially available 4-nitrophenyl glycopyranosides have a low molarextinction coefficient under these conditions that hamper their use asmodel compounds at pH values below 9. As a consequence, rapid catalystscreening in 96-well plate format using such substrates is limited andresults in high uncertainty for the evaluation of the catalystperformance due to the resulting small apparent absorbance changes,small extinction coefficients, and large errors of the associated data.To overcome this obstacle, at least two approaches may be utilized: theuse of derivatized nitrophenyl glycosides with large extinctioncoefficients suitable to follow hydrolysis reactions at nearphysiological pH by UV/Vis spectroscopy, and/or the use of ‘real’saccharides instead of phenylglycoside model compounds after furthermodification of the current assay. Toward the development of functionalenzyme models, both approaches were followed and the results aresummarized below.

Example 4 Discrimination of 2′-chloro-4′-nitrophenyl GlycopyranosidesDuring Catalytic Hydrolysis at Physiological pH

With substrates 9a-i on hand, their hydrolysis was initially studiedunder the same conditions as outlined above for the hydrolysis ofp-nitrophenyl glycopyranosides, i.e. at alkaline pH. While the catalyticrate constants are of similar order of magnitude, the uncatalyzedhydrolyses of 2′-chloro-4′-nitrophenyl glycopyranosides (e.g.k_(non)=0.5-3.3×10⁻⁴ [min⁻¹ M⁻¹], 50 mM CAPS buffer, pH 10.5, 30° C.)are considerably faster than the uncatalyzed hydrolysis reactions ofp-nitrophenyl glycopyranosides (Table 1) rendering all complexes lessefficient as catalysts and the differences in catalyst proficiencysmaller. This observation accounts for the decreased stability of theglycosidic bond after introduction of the chloro-substituent in theortho position of the aglycon. However, the decreased stability of theglycosidic bond in the 2′-chloro-4′-nitrophenyl glycopyranosides doesallow the investigation of the catalyst performance at near neutral pHvalues as noted above.

Along these lines, the proficiency of the selected complexes towardglycoside hydrolysis was evaluated in 50 mM HEPES buffer at pH 7.50 and30° C. Typically, lag times around 200 min were observed prior to thestart of the catalytic hydrolyses that may indicate substratedeprotonation, distortion or inversion of the glycon upon interactionwith the catalyst. Data collection over 9-12 h allowed the determinationof all kinetic parameters after conversion of the observed absorbancedata into product concentrations and application of the Michaelis-Mentenmodel (Table 2).

TABLE 2 Kinetic parameters for the catalytic hydrolysis of 9a-f, i at pH7.5 and 30° C. Entry S cat k_(cat) × 10⁻³ [min⁻¹] K_(M) [mM]k_(cat)/(K_(M) × k_(non))  1 9a 1 1.51 25.7 43,000  2 S-1 1.74 30.042,000  3 R-1 1.98 38.3 38,000  4 9b 1 0.42 25.6 62,000  5 S-1 0.33 22.855,000  6 R-1 0.26 17.3 57,000  7 9c 1 0.37 15.6 51,000  8 S-1 0.28  8.868,000  9 R-1 0.71 31.6 48,000 10 9d 1 5.49 73.0 35,400 11 S-1 4.55 58.437,000 12 R-1 3.28 43.2 36,000 13 9e 1 0.07  4.5 66,000 14 S-1 0.06  2.1121,000  15 R-1 0.06  2.2 118,000  16 9f 1 1.10 30.7 50,000 17 S-1 1.6747.7 49,000 18 R-1 1.26 45.0 39,000 19 9i 1 0.53  4.0 60,000 20 S-1 0.49 4.8 47,000 21 R-1 0.24  1.9 58,000 where k_(non), 9a = 1.3 × 10⁻⁶[min⁻¹ M⁻¹]; k_(non), 9b = 2.6 × 10⁻⁷ [min⁻¹ M⁻¹]; k_(non), 9c = 6.7 ×10⁻⁷ [min⁻¹ M⁻¹]; k_(non), 9d = 2.1 × 10⁻⁶ [min⁻¹ M⁻¹]; k_(non), 9e =2.4 × 10⁻⁷ [min⁻¹ M⁻¹], k_(non), 9f = 7.1 × 10⁻⁷ [min⁻¹ M⁻¹]; k_(non),9i = 2.2 × 10⁻⁶ [min⁻¹ M⁻¹]

Example 5 Performance of S-1 During Hydrolysis of Epimericα-Glycopyranosides

Chiral catalyst S-1 discriminates epimeric 60 -glycosides 9a, 9c and 9eby catalyzing their hydrolysis with significantly different rates (FIG.2). The rate for the catalyzed hydrolysis of 9a is more than 6-foldhigher than for 9c and 29-fold higher than for 9e pointing at asignificant influence of the hydroxyl groups at C-2 and C-3 in theglycon of the substrates during metal complex-catalyzed hydrolysis.Substrates with hydroxyl groups trans to each other (9c, 9e) promoteslower hydrolysis than a substrate with hydroxyl groups cis to eachother (9a). This observation correlates with known metal complexcoordination abilities to trans-diols (weak) and cis-diols (strong) inpyranosides. To account for uncatalyzed background reactions anddifferent substrate affinity toward a complex, the catalyst proficiency(k_(cat)/K_(M)×k_(non)) was calculated (FIG. 3).

The known lability of the α-glycosidic bond in 9a causes its uncatalyzedhydrolysis (k_(non)) to be about an order of magnitude faster than for9c and 9e. Thus, the catalytic proficiency of chiral complex S-1decreases in the order of 9e >9c >9a showing a distinct discriminationof epimeric glycosides by a metal complex at near neutral pH. Forcomparison, symmetric complex 1 shows an overall lower proficiency tohydrolyze the epimeric substrates with negligible differences (1.5-foldor less).

Example 6 Performance of S-1 During Hydrolysis of α- andβ-Glycopyranosides

The rate of the chiral complex S-1 catalyzed hydrolysis of α-mannoside9a is about 5-fold higher than for β-mannoside 9b, while the reversetrend is observed for gluco- and galactosides where the β-glycosides arehydrolyzed 16-fold (9d) and 28-fold (9f) faster than their corresponding60 -glycosides (Table 2, entries 2 & 5; 8 & 11; 14 & 17). As theuncatalyzed hydrolyses of the manno- and galactosides differ by an orderof magnitude within each α-/β-pyranoside pair (Table 2, footnote), thecatalytic proficiency of the catalyst for the hydrolysis of therespective α- and β-glycosides do not reflect the discrimination itsability, but reveal a rather equivalent catalytic proficiency near50,000 instead. Substrate 9i behaves likewise.

By contrast, the uncatalyzed reactions of the α- and β-glucosides 9e and9f are of the same order of magnitude and the chiral complexS-1-catalyzed hydrolyses differ by almost 1.5 orders of magnitude or28-fold (FIG. 4; Table 2, entries 14 & 17). This observation translatesinto a 3-fold higher proficiency of chiral complex S-1 to hydrolyze 9eover 9f due to higher substrate affinity of 9f over 9e for chiralcomplex S-1, and demonstrates the ability of a metal complex todiscriminate α- and β-glycosidic bonds notably close to physiologicalpH. This finding is of particular significance for catalyst development,use of biomass or its transformation into fine chemicals and fuel due tothe abundance of α- and β-glucopyranosyl moieties as building blocks innatural products and oligosaccharides including cellulose and starch.

Example 7 Performance of R-1 During Hydrolysis of Glycopyranosides

As elaborated previously, the overall catalyst performance correlates tothe intramolecular Cu . . . Cu distance in the metal complex core. Assimilar rates of the catalyzed substrate hydrolyses are observed forchiral complexes S-1 and R-1 (Table 2), the chirality of the complexesis, however, unlikely to have a profound contribution to the observeddiscrimination of the epimeric or anomeric model compounds by chiralcomplex S-1 and is echoed by chiral complex R-1. Instead, theconfiguration of the hydroxyl groups in the glycon of the substratepresents itself as a rationale for the observed glycosidediscrimination. For simplicity, the following discussion on themechanistic insights is consequently limited to chiral complex S-1.

Example 8 Putative Mechanism of the α-Glycoside Hydrolysis

The high catalytic proficiency of chiral complex S-1 for the hydrolysisof glucopyranosides 9e-f over those of 9a-d indicates again differentinteractions of the catalyst with the glycon of the substrates. As theepimeric substrates are overall only different in their configuration atC-2 and C-4, respectively, we proposed that the higher acidity of thehydroxyl group at C-2 over that of the hydroxyl group at C-4 and itsproximity to the anomeric center promote substrate deprotonation andcoordination to the metal complex as pre-requisite for catalytichydrolysis to occur. The coordination is consequently stronger when adeprotonated cis-diol structure (9a) is participating in metal complexchelation and weaker when a trans-diol structure (9c or 9e) is presentaccounting for above described observations.

For experimental evidence, the catalytic hydrolysis of substrates 9g and9h was evaluated. Both substrates are methylated in the glycon at thehydroxyl group at C-2 preventing deprotonation at this position uponinteraction with the catalyst, while weaker hydrogen-bondinginteractions are still enabled. The hydrolysis of 9g and 9h isunsuccessful with any studied catalyst both at pH 7.5 and in alkalinesolution at pH 10.5 indicating that all substrates indeed coordinate tothe catalysts over a deprotonated hydroxyl group at C-2. A putativemechanism for the hydrolysis of α-glycoside by chiral complex S-1 at pH7.5 is deduced from the described experimental observations, anddepicted for the hydrolysis of 9a below.

Upon dissolving chiral complexes S-1 and R-1 in solution, a binuclearspecies I is formed that coordinates the glycoside substrate underrelease of water and protons resulting in half-acetal formation (Scheme5, species II). Distortion of the configuration may then yield astructure similar to the transition state proposed for enzymaticglycoside hydrolyses encompassing substrate distortion to a half-chairconformation, sp²-character of the anomeric C-atom, partially positivelycharged endocyclic O-atom and lengthening of the glycosidic bond (Scheme5, species III). Models suggest a twisted boat-like structure for asimilar species derived from β-glycosides requiring furthercomputational analyses in future efforts. Hydration of the binuclearcopper species and protonation of the sugar by solvent molecules mayrelease the coordinated hydrolysis products to reform species I andclose the catalytic cycle.

Example 9 Discrimination of Disaccharides During Catalytic Hydrolysis

In order to apply the catalysts to natural carbohydrates, all complexeswere explored in their ability to hydrolyze the disaccharides maltose(9), cellobiose (10), and lactose (11) (FIG. 5a ). Disaccharides 9 and10 differ in the nature of their glycosidic bonds, and 10 and 11 areepimers in their non-reducing sugar moiety. All disaccharides werehydrolyzed at 60° C. at pH 8 over 24 h in presence and absence of 10 mol% of symmetric complex 1, chiral complex S-1, and chiral complex R-1,respectively. The amount of remaining starting material was thenquantified by HPLC analysis (FIG. 5b ). Evidence for catalystdestruction or sugar oxidation were not apparent under the elaboratedconditions. The composition of the complexes is similar to that at pH7.5 with slightly different amounts of the major catalytically activespecies.

All experiments were conducted in 100 mMN-[Tris(hydroxymethyl)methyl]-2-aminoethanesul-fonic acid (TES) bufferat pH 8.0 and 60° C. in triplicate, and the obtained data were averaged.The substrate stock solutions were prepared from maltose (9), cellobiose(10) or lactose (11). Typically, 25-30 mg (73-83 mmol) of thedisaccharides were dissolved in 1 mL nanopure water yielding 70-83 mMsubstrate stock solutions. A constant 50 μL aliquot of each stocksolution was then used for all experiments. 5 mg of the binuclear metalcomplexes were dissolved in 10 mL of buffer solution yielding 0.73 mMstock solutions. The catalyst stock solutions were then used in constant450 μL aliquots for all experiments. The 50 μL aliquot of the substratestock solutions were diluted with 450 μL nanopure water, buffer orcatalyst stock solutions and heated to 60° C. After 24 h, the solutionswere cooled in ice, and 100 μL aliquot was taken and 30 mM sodiumsulfide solution added. After centrifugation for 5 min at 8.5 g, thesupernatant was filtered and subjected to HPLC analysis.

All experiments were conducted on a Shimadzu HPLC with aRezex-Carbohydrate Na⁺ (8%) column 300×7.8 mm and 50×7.8 mm guard column(Phenomenex) using nanopure water as eluent isocratic with a flow rateof 0.4 mL/min at 80° C. and ELS detection. The filtered samples werediluted with an equal volume of 50 mM acetic acid buffer at pH 5.0. Theresulting solution was subjected to analysis in 25 μL aliquots, and theelution was monitored for 30 min. Disaccharides elute under theseconditions between 14-18 min, monosaccharides elute between 20-24 minand the TES buffer elutes at 22.5 min. The area of the peaks in thechromatograms was integrated using the software supplied by Shimadzu.The percentage of hydrolysis for each sugar aliquot was determined bycorrelation of the area in the respective sample to a reference samplesin water.

The data reveal indifference of symmetric complex 1 to hydrolyze theselected disaccharides in any significantly higher amount than buffersolution alone. By contrast, chiral complex S-1 hydrolyzed twice as muchof α-glycoside 9 as buffer, symmetric complex 1, or chiral complex R-1.Likewise, chiral complex R-1 hydrolyzed up to 2.5-fold moreβ-glycosides10 and 11 than buffer solution or another catalyst. The findingindicates stereoselective discrimination of the glycosidic bonds indisaccharides by the chiral catalysts with a preference of chiralcomplex S-1 for α-glycosidic bonds and of R-1 for β-glycosidic bonds.

Example 10 Performance of Binuclear Zn (II) Complexes During Hydrolysisof Glycopyranosides

FIG. 6 illustrates catalyzed hydrolysis of2-chloro-4-nitrophenyl-α-D-mannopyranoside at pH 7.5 in 50 mM HEPESbuffer, with Cu₂bpdpo (red) (k_(cat)=8.1±0.9 10⁻⁴ min⁻¹; K_(M)=12.0±2.3mM) and Zn₂tpdpo (blue) (k_(cat)=6.4±0.6×10⁻⁴min⁻¹; K_(M)=11.0±1.6 mM;k_(non)=4.4×10⁻⁷min⁻¹ M⁻¹).

Whereas, the compounds and methods have been described in relation tothe drawings and claims, it should be understood that other and furthermodifications and formulations, apart from those shown or suggestedherein, may be made within the scope of this invention.

What is claimed:
 1. An enantiopure asymmetric chiral binucleartransition metal (II) complex for discriminating α-glycosidic bonds ofsaccharides in aqueous solutions, wherein the formula of saidenantiopure asymmetric chiral binuclear transition metal (II) complexis:


2. An enantiopure asymmetric chiral binuclear transition metal (II)complex for discriminating β-glycosidic bonds of saccharides in aqueoussolutions, wherein the formula of said enantiopure asymmetric chiralbinuclear transition metal (II) complex is:


3. A method of hydrolysis of saccharides and glycosides, saidmethod_comprising the steps of: discriminating β-glycosidic bonds ofsaccharides in aqueous solutions using an enantiopure asymmetric chiralbinuclear transition metal (II) complex, wherein the formula of saidenantiopure asymmetric chiral binuclear transition metal (II) complexis:


4. The method of claim 3 further comprising the step of discriminatingsaid β-glycosidic bonds of saccharides in aqueous solutions at a pH ofabout 7.5 using said chiral binuclear transition metal (II) complex.